Replication errors are manifested as single DNA base mispairs, or insertion/deletion loops created by replication slippage events. One pathway responsible for this repair is the MutSLH mismatch repair pathway of Escherichia coli. MutSLH-like repair is conserved from bacteria to humans. In humans, deficiencies in this repair are linked to hereditary nonpolyposis colon cancer (HNPCC). One limitation of the mismatch repair pathway is its apparent inability to recognize looped mismatches greater than 4 bases. There is abundant in vivo evidence in eukaryotes that these larger loops are efficiently corrected, but this activity is not the result of the eukaryotic MutSLH homologues. These data suggest that a second repair pathway unique to eukaryotes which targets larger insertion/deletion loops for repair. The aim for this proposal is to characterize biochemically the DNA and protein factors necessary for this novel repair in vitro. This work will be facilitated by the use of Saccharomyces cerevisiae, whose easy genetic manipulation and wealth of in vivo repair studies will allow for assignment of physiological relevance to the biochemical results obtained. Ultimately, the information derived from these experiments will be used to determine if loop repair is completely separate from MutSLH-like repair or if the two mismatch repair pathways share common elements in later steps of the repair process.